Will Petraeus cut costs in Afghanistan as he did in Iraq?

After replacing Gen. Stanley McChrystal with Gen. David Petraeus in Afghanistan, President Obama can use this opportunity to explain his current thinking about the trade-off between containing the war's costs and his goal of leaving no safe-haven for al Qaeda.

As an officer astute about politics, Petraeus knows the new politics of austerity in Washington. Congress is in little mood to add to the debt. Afghanistan now costs more than Iraq, or some $70 billion this year, even as a troop surge goes on.

Mr. Obama warns he has a budget limit for Afghanistan. “We simply cannot afford to ignore the price of these wars,” he says. He seeks a balance between domestic priorities, especially the economy, and the threat of violence from Al Qaeda or its affiliates in Afghanistan as well as Pakistan.

Defining victory, however, remains illusive and divisive among Obama’s security team, one reason for McChrystal’s ill-spoken words to a reporter. Rather than wait until December, Obama should use this change of command to give a clear explanation of the current trade-offs between the war’s costs and the evolving security threats.

Is he still set on denying a haven for Al Qaeda, as promised – even if, for instance, the coming offensive in the Taliban stronghold of Kandahar fails and drags the war into 2012 and beyond?

The president dislikes an open-ended commitment to the war. And he uses the threat of withdrawal to pressure Kabul to quickly boost its forces and the economy. But squaring his goal of securing that country enough to prevent another 9/11 with the budget pressures back home will require Obama to keep a running dialogue with the American people. Since last year, they have largely found the war not worth fighting.

It takes more than one good general to win a war. The home front is a battleground, too.